There are several types of light emitting diodes (LEDs) that emit in the visible (red) portion of the spectrum. Many of these are made with a crystal structure composed of aluminum, gallium and arsenic to form aluminum gallium arsenide (AlGaAs). AlGaAs LEDs can be classified into two groups: those with opaque substrates (OS) and those with transparent substrates (TS).
FIG. 1 shows an example of a prior art OS AlGaAs LED. An epitaxial layer 102 of p-doped AlGaAs and an epitaxial layer 101 of n-doped AlGaAs are grown on top of a p-doped gallium arsenide (GaAs) substrate 103. Light is generated as a result of current flowing through the junction of layers 101 and 102. The surface area of the Os AlGaAs LED, for example, is ten mils by ten mils. Layer 101 is, for example, twenty microns thick, layer 102 is, for example, twenty microns thick and substrate 103 is, for example, eight mils thick.
The OS AlGaAs LED shown in FIG. 1 has a single heterojunction (SH). That is, the bandgaps of layers 101 and 102 are chosen such that light generated in layer 102 travels through layer 101, without being absorbed. Layer 102 is thus called the active layer, and layer 101 is called the window layer.
The primary disadvantage of the OS AlGaAs LED shown in FIG. 1 is its inefficiency. Roughly 90% of the generated light is absorbed by substrate 103.
FIG. 2 shows an example of another prior art OS AlGaAs LED. An epitaxial layer 203 of n-doped AlGaAs, an epitaxial layer 202 of p-doped AlGaAs and an epitaxial layer 201 of p-doped AlGaAs are grown on top of an n-doped gallium arsenide (GaAs) substrate 204. Light is generated as a result of current flowing through the junction of layers 202 and 203. The surface area of this OS AlGaAs LED, for example, is ten mils by ten mils. Layer 201 is, for example, ten microns thick, layer 202 is, for example, one and a half to two microns thick, layer 203 is, for example, ten microns thick and substrate 204 is, for example, eight mils thick.
The OS AlGaAs LED shown in FIG. 2 has a double heterojunction (DH). That is, the bandgaps of layers 201, 202 and 203 are chosen such that light generated in layer 202 travels through layers 203 and 201, without being absorbed. Layer 202 is thus the active layer, and layers 201 and 203 are window layers.
The OS AlGaAs LED shown in FIG. 2 is more efficient than the OS AlGaAs LED shown in FIG. 1. However, much of the generated light is absorbed by substrate 204, making the OS AlGaAs LED shown in FIG. 2 still quite inefficient.
FIG. 3 shows an example of a prior art TS AlGaAs LED. An epitaxial layer 303 of n-doped AlGaAs, an epitaxial layer 302 of p-doped AlGaAs and an epitaxial layer 301 of p-doped AlGaAs are grown on top of a gallium arsenide (GaAs) substrate (not shown). The substrate is etched away leaving only layers 301, 302 and 303. Light is generated as a result of current flowing through the junction of layers 302 and 303. The surface area of the TS AlGaAs LED, for example, is ten mils by ten mils. Layer 301 is, for example, thirty microns thick, layer 302 is, for example, one and a half to two microns thick and layer 303 is, for example, one hundred microns thick.
The TS AlGaAs LED shown in FIG. 3 has a double heterojunction (DH). That is, the bandgaps of layers 301, 302 and 303 are chosen such that light generated in layer 302 travels through layers 303 and 301, without being absorbed. Layer 302 is thus the active layer, and layers 301 and 303 are window layers.
The TS AlGaAs LED shown in FIG. 3 is much more efficient than the OS AlGaAs LEDs shown in FIGS. 1 and 2. There is no GaAs substrate, thus no light gets absorbed there, and much more of the generated light escapes the LED. However, producing a TS AlGaAs LEd such as that shown in FIG. 3 is difficult and expensive. Therefore, in the prior art, an AlGaAs LED, with efficiency comparable to the LED shown in FIG. 3, may not be produced inexpensively.